New Thermooptical Measurement Method and a Comparison with Other Methods

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Abstract
A simple and sensitive method of measuring the thermally induced index changes arising from absorption of a laser beam in a low-loss material is described. The sample is placed outside the laser cavity, but at the position of minimum radius of curvature of the wavefront, which is a confocal distance behind the waist. It is estimated that the method is sensitive enough to measure absorption coefficients of the order of 5 × 10−6 cm−1 and it is shown experimentally to have good accuracy on low-loss materials. A detailed comparison of sensitivity and accuracy estimates is given for the various published thermal-lens methods for measuring low absorption coefficients.